How to Choose a Camshaft and Valvetrain for Forced Induction

If ever you’ve had to choose the right camshaft for your engine you understand how tricky it can get. It boils down to knowing what you want the engine to do and how the vehicle will be driven most of the time. Where cam selection gets dicey is when you add power boosters like supercharging, turbocharging, or nitrous. What then?

How power is made in an engine is raw physics according to Dave Akard at Burbank Speed & Machine. There are no free lunches. He stresses making usable power you can feel at your backside is hard to do. Power does not come cheap and it has never come easily no matter what automotive buff books and websites will tell you. Oh sure, you can get modest power increases with manifold and carb swaps, more robust heads, better exhaust scavenging, and a hotter cam. You can get quicker by changing axle ratio or going to a close-ratio transmission. However, when it comes to making real power, you’ve got to increase compression or resort to the boosted induction.

How much power can you get from forced induction? Under optimum conditions you can experience as much as a 50 percent increase. However, this is the real world and your numbers are likely to be less. You have to go the extra mile to even get close to that 50 percent increase.

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When you opt for power adders like superchargers, turbochargers, or nitrous you are changing your engine’s dynamic. More importantly, your engine has to be ready for it. You are going from naturally aspirated to forced induction where incoming air is forced into the cylinders under pressure known as boost, which changes how the engine breathes and makes power. You don’t want to run forced induction with the dynamics of a naturally-aspirated engine. It has everything to do with valve timing events. And then there’s bore and stroke along with rod ratio that determines whether or not boost is even worth the investment. It also has to do with your engine’s bones and whether it can handle the boost safely. Too much boost with an unprepared engine can mean disaster.

Compression ratio is a huge factor with forced induction. Too much compression with forced induction can destroy an engine in nanoseconds at wide-open throttle because detonation happens quickly. Compression ratio and forced induction are a series of tradeoffs. With increased compression you get more horsepower and torque. With increased boost comes more horsepower and torque. When you combine the two, engine life hangs in the balance. With supercharging and turbocharging compression must be conservative in the 8.5:1 to 10.0:1 range if you’re going to run 5 to 8 pounds of boost and 92-octane pump gas. Ignition timing and air/fuel ratio should also be conservative and programmed by a qualified tuner.

An engine has to be fully prepared for boosted induction or you’re just wasting time and money. Cast or hypereutectic pistons will not survive excessive amounts of boost because they will not withstand the excessive combustion temperatures you get with boost.

When you go from naturally aspirated to supercharging, turbocharging, or nitrous you also have to go to a camshaft designed to work well with forced induction. Once you make that decision, you have to fine-tune valve timing events to work well with the type of forced induction you intend. Supercharging performs differently (more quickly) than turbocharging, hence the need for a different cam profile. Turbo lag is why you need a different cam profile. Nitrous is yet a completely different element because it comes into play at wide-open throttle and full power. Nitrous and supercharger cams generally employ very similar profiles because both do their best work at high rpm.

Cam selection really depends on who you talk to about blower cams versus stock cams. Some seasoned builders and racers swear by stock camshaft grinds for blower applications because there’s not as much valve overlap and wider lobe separation with a stock grind. The concern with valve overlap and narrow lobe separation is losing precious boost pressure via the exhaust valve during overlap.

One expert we know in this field is Richard Holdener, who has a tremendous amount of dyno testing experience, who tells us cam duration beyond stock is used in a wide variety of blower applications to determine effective engine speed. He adds that with a normally aspirated engine, cam specifications, primarily duration, can be used along with intake manifold runner length to determine where the engine will produce peak horsepower and torque. You can’t just look at it as cam and blower, but the entire engine package, including displacement, rod ratio, cylinder heads, induction, and exhaust.

We learn via experience it takes more than just cam profile to make power. There are many other engine elements that work along with cam profile. Cylinder heads, port configuration, valve size, chamber shape and size, compression, header tube sizing and configuration, and even axle ratio affect how your boosted engine will perform where the rubber meets the road.

Holdener adds that moving the torque curve higher in the rpm range will normally produce a higher peak power level, but low-end torque will suffer in the balance. You might suspect that this tradeoff has a negative effect on power once you add boost, but Holdener’s dyno-testing experience has been known to prove otherwise. He has experienced extraordinary results with forced induction and a variety of cam profiles.

Cam selection has to wait until you’ve chosen a specific type of forced induction. Positive-displacement superchargers work best at lower-rpm ranges where they deliver an abundance of torque, however, they tend to taper off at high rpm. There are always exceptions to this rule depending on the supercharger and how the engine is tuned.

Centrifugal superchargers do their best work at high rpm due to the very nature of their design. The faster we spin them the more boost they make. Nitrous requires a cam profile similar to that of a centrifugal supercharger—which is high-rpm friendly.

These factors play into the type of cam profile you’re going to select. Because we’re forcing a bunch of air into the cylinders under pressure, how we manage exhaust scavenging is everything to power. You don’t want the air/fuel mixture to be contaminated with spent exhaust gases from the previous power cycle that haven’t left the chamber. You also want more lift and duration on the exhaust side to aid scavenging. Add to that the need for wider lobe separation to reduce valve overlap and lost power. It is a fine line between valve overlap and exhaust lift and duration.

Shane Pulido of Crower Cams tells us “There are many different avenues you can go down when it comes to choosing a cam designed for boosted engines, but the basic premise is a wider “Lobe Separation Angle” (LSA), which keeps boost pressure from going out the exhaust during valve overlap.” He adds at the same time you’re going to want more duration and lift on the exhaust side when the exhaust valve finally opens.

Shane goes on to say, “Supercharging and nitrous are the closest in nature with more numbers between intake/exhaust duration. For example 240/255-degrees duration (valves off their seats) at 0.050 inch with a wide lobe separation angle somewhere between 112-120 degrees.” However, he adds, “This is based on many variables, such as cylinder head flow numbers, chamber size and shape, valve size and shrouding, compression ratio, and the rest of it.” We cite these factors because the overwhelming amount of cylinder pressure needs to get out.”

When we asked Shane about turbocharging, he had this to say: “Turbocharging is a whole different approach when it comes to planning cam profile. However, here at Crower we have learned over the past 50 years to increase in intake duration while shortening up exhaust duration, which contradicts what we’ve said about superchargers.” He adds, “For example, with turbocharging, you going to want 240/235-degrees duration at 0.050 inch with a wide lobe separation angle of 112-118 degrees. Our logic behind this is to close the exhaust valve and keep the intake valve open longer to get a good healthy cylinder charge before compression and power strokes.” Shane further adds this will also help spin the turbo aggressively for a faster spool up. And with that faster spool up you get more turbo boost quickly along with the resulting power.

Before we get into how to choose a boost cam, it is important to consider the most basic choice first—flat-tappet versus roller. For a boosted application you want crisp camshaft performance with more linear valve motion, which is only available from a roller cam. Roller tappets follow the lobe with greater precision than flat tappets.The roller advantage is obvious over flat-tappet when it comes to having a user-friendly lobe profile. With a roller cam, you can position the lobes to get the valve timing events exactly where you want them. Here, we’re showing lobe separation angle between intake and exhaust lobes on a Crower roller cam.A roller cam enables you to customize valve timing events with either an off-the-shelf cam or a custom grind from Crower Cams. Crower can custom grind a cam to any profile you desire based on what you want a boosted engine to do.This flat-tappet hydraulic Crower camshaft demonstrates the limitations of a flat-tappet cam. It is physically impossible to have an aggressive cam profile for a boosted application without the lifter being wedged into the lobe.Camshaft identification can typically be found at the tail end of the cam. Cam manufacturers all have different approached to identification.A typical cam card will have all of the specifications as shown here on this Howard Cams card. When you degree the cam, it is an opportunity to compare the card to the actual specifications.Another cam card example offers printed specifications. All necessary specifications are here to properly degree a cam and know what you have, then, make any necessary changes to valve timing at the crank sprocket.We’re degreeing a cam in a big-block Chevy to see how it compares with the manufacturer’s cam card. We’re also checking true piston top dead center, which is what you must know while degreeing a cam. True top dead center is when the piston is at top dead center and the connecting rod straight up at 12 o’clock.This is true top dead center where the piston stops during crank rollover, which is known as piston dwell time. Piston dwell time is affected by connecting rod ratio, which is stroke versus the rod’s length. The greater the dwell time the more power you can make.These are true handwritten camshaft specifications, which are the result of cam degreeing. The builder reviews these numbers and how they compare to the cam card. When you factor these numbers in with rod ratio and true top dead center it enables you to know what you have from the block deck down.Cam selection in a boosted application means filling the cylinder with as much air and fuel as possible under pressure. Pressure comes immediately with a supercharger. What makes a turbo application different is the lag you experience from a turbo, which is why turbo intake and exhaust valve timing is different than supercharger and nitrous.Another issue with forced induction is compression ratio. Power comes from compression increases or increases in boost. By increasing cylinder pressure via compression or boost you get more power. However, should you overdo either you run the risk of engine damage from detonation. Dished pistons (negative dome) yield less compression.Domed pistons (positive dome) yield greater compression because you are reducing chamber size above the piston. A domed piston does not always mean higher compression. With a larger chamber comes the need for a domed piston to fill the chamber. You want good quench from all that area (arrows) between the chamber and piston.This is a smaller high-swirl combustion chamber, which yields greater quench (arrows) above the piston for more thorough combustion and power. Quench should average 0.038 to 0.043 inch between the piston and cylinder head with the head gasket installed. You don’t want any more than 0.060-inch clearance with the head gasket installed.These dated shovel-style chambers in an old Ford head offer terrible quench compared to modern heads with smaller high-swirl chambers. Open chambers like these tend to detonate at higher pressures because there’s very little quench area.Chevrolet had the right idea back in the day with its big-block heads. Plenty of quench for more complete combustion and less risk of detonation. Chevrolet put the spark plug smack in the middle of the chamber for a more uniform light-off.Another issue with cam selection for a boosted application is rod ratio, which is rod to stroke. If you have a 4.500-inch stroke and a 6.500-inch rod (center-to-center) you have a rod ratio of 1.44:1. For a boosted application, you want as much piston dwell time as possible at each end of the bore. Aim for the most rod length you can stuff into the block with the existing stroke. Optimum rod ratio is 1.75:1.By the same token, go for as much stroke as you can fit into the block, but be careful—too much stroke can be a bad thing. On positive side, stroke gets you increased torque.When you’re calculating compression ratio for a supercharged application don’t forget head gasket thickness, which must be figured into the equation. Also remember you’re going to need a head gasket engineered for a supercharged application.Rocker arm ratio affects valve lift. These Jesel race-ready shaft-mounted rocker arms on a small-block Chevy are 1.6:1, which means valve lift is 1.6 times greater than lobe lift at the cam. Any time you increase rocker arm ratio, you increase how far the valve opens.When you select a boost-profile cam, valvespring pressure must follow lobe profile. Ideally, you will order a cam kit with matched components compatible with the cam. If you choose a Crower product, consult with a Crower customer service technician who can get you dialed in with the right components.It is suggested you choose an adjustable timing set where you can dial in valve timing in your boosted application. You can advance or retard valve timing to achieve optimum power. Adjustment occurs at the crank.Adding power can be enhanced by also freeing up power trapped by internal friction. Opt for Torrington bearings and roller technology to reduce internal friction.Once you have installed a supercharger, turbocharger, or nitrous you should look to a seasoned engine tuner, such as Ray McClelland of Full Throttle Kustomz (FTZ), for a full-scale dyno tune. Not only does a professional dyno tune net you the most power possible, it keeps your engine out of trouble. Too much timing and not enough fuel can be deadly.These broken piston rings are the result of detonation from a bad tune. You want safe ignition timing and fuel curves, which come from a professional dyno tune.A positive displacement supercharger (Roots blower) calls for a different cam profile than a centrifugal due to when these blowers do their best work. These Weiand positive displacement superchargers from Holley deliver excellent low- to midrange torque as well as seamless horsepower at high rpm.STS Turbochargers from Holley overcome the challenges of traditional turbo installations because the turbo can be mounted in a remote location such as in place of the stock muffler or down low in the chassis. Your cam profile should be turbo-specific.Nitrous applications call for a cam profile similar to supercharging based on when centrifugal superchargers and nitrous do their best work, which is at high rpm. It is suggested you contact Crower’s tech staff to achieve the most optimum cam selection. Cam selection has to be fine-tuned based on the level of performance desired.